This invention relates to a novel process for preparing carbamates.
Carbamates are a basic organic compound and a useful substance as a material for pharmaceuticals and agricultural chemicals, or as a reagent for qualitative analysis of organic compounds (used as a crystalline derivative for confirmation of alcohols or phenols). Particularly, its ethyl ester is also called "urethane" and has various uses as a raw material of a sleeping drug.
As a process for preparing carbamates, a process of heating alcohol with urea has been known as a conventional technique. For example, in U.K. Patent No. 982,785, there has been disclosed a process in which an aliphatic monohydric alcohol having up to 8 carbon atoms and urea are mixed in a proportion of at least two moles of said alcohol per mole of urea, and the mixture obtained is heated for not more than 1.5 hours in the absence of a catalyst at a temperature in the range of 180.degree. to 230.degree. C. to react said alcohol with urea, during which a venting rate of ammonia gas which is a by-product from the "mixture which is being heated" is controlled to maintain a pressure of the system within the vapor pressure of said "mixture which is being heated". Also, in U.S. Patent No. 3,013,064, there has been disclosed a process for preparing a lower alkyl carbamate by reacting a lower alkanol having 1 to 3 carbon atoms with urea at a temperature of 130.degree. to 155.degree. C. and a pressure of 3 to 10 atmospheres while withdrawing vapor occurred from the reaction zone.
On the other hand, a process of reacting chloroformate with ammonia and a process of reacting alcohol with phosgene and ammonia (literature name: Annalen, vol. 10, p. 284) have been also known as a conventional technique.
As a process for preparing carbamates of which a starting material is carbonic acid diester, as disclosed in Japanese Unexamined Patent Publication No. 503627/1989, there has been known a process in which said carbonic acid diester are reacted with amines in the presence of amidines as a strong organic base catalyst at a temperature of 0.degree. to 100.degree. C. and at a pressure of 1 to 10 bar.
Further, in Japanese Unexamined Patent Publications No. 14745/1977 and No. 163528/1979, there has been disclosed processes in which a hydroxide of an alkali metal or an alkali earth metal or an alcoholate is used as a catalyst.
However, the above process of reacting alcohol with urea requires high temperature or high pressure, whereby installation for manufacture becomes complicated, and the process of reacting chloroformate with ammonia has problems that chloroformate is expensive and a corrosive hydrochloric acid is by-produced. Further, in the process of reacting alcohol with phosgene and ammonia, extremely toxic phosgene should be handled, whereby there is a problem in the point of safety, and also a corrosive hydrochloric acid is by-produced similarly as in the case of the process of reacting chloroformate with ammonia.
The process of using a strong organic base as a catalyst involves problems that the catalyst itself is very expensive and steps for purifying carbamates which are the desired products become complicated. Further, even in the process of using a hydroxide of an alkali metal or an alkali earth metal or an alcoholate as a catalyst, it involves a drawback that steps for purifying carbamates such as separation and removal of a catalyst from a reaction product become complicated.
Accordingly, the processes for preparing carbamates of the prior art are not necessarily industrially satisfied.
The conventional processes for preparing carbamates involve various problems, as described above, that high temperature and high pressure are required for reaction, a corrosive hydrochloric acid is by-produced and steps for purifying carbamates are complicated.